How about all the stuff from JFK’s assassination? I think that has a holding period of at least 75 years.
It isn’t so much the materials from the JFK assassination that have been supressed, it is materials from the JFK administration (including tapes, it was JFK who installed the White House taping system that later brought down President Nixon).
Some of the LBJ tapes have come out and they were damning. He secured censorship from the press (no negative reporting) in exchange for letting a bank merger go through, for example.